Influence mechanisms of underwater hyperbaric environment on nitrogen behavior, phase evolution, and mechanical properties of high nitrogen steel repaired by underwater laser direct metal deposition

Abstract

Nitrogen outgassing and nitrogen pore formation are tricky problems inherent in the fabrication/repair of high nitrogen steel (HNS). In the current research, HNS powder with a nitrogen content of 0.42 wt% was used to repair damaged HNS plates by the underwater laser direct metal deposition (UDMD) technique at an ambient pressure of 0.3 MPa (sample U1). Under the same process parameters, the DMD repaired sample (sample A1) in the ground environment served as a comparison. The influence mechanisms of the underwater hyperbaric environment on the nitrogen behavior, interface crystallographic characteristics, and mechanical properties were clarified. Results showed that nitrogen outgassing and nitrogen pore formation occurred in sample A1. In sample U1, the elevated ambient pressure increased the nitrogen solubility in the molten pool from 0.628 wt% to 1.088 wt%, thus eliminating porosity defects. In addition, there was a higher content of Kurdjumov-Sachs (KS) and Nishiyama-Wassermann (NW) interfaces (95%) in sample A1. Conversely, the water cooling inhibited the austenite shear transformation period at the relatively low temperature of sample U1. Hence, lower KS/NW interfaces (56%) were obtained due to the changed austenite nucleation and growth mechanism. Benefitting from the denser dislocation density and finer grain size caused by the underwater environment, the comprehensive mechanical properties of defect-free sample U1 were superior to those of sample A1. The nitrogen pores in sample A1 did not weaken the tensile strength but did seriously impair the impact toughness and ductility.